Researchers sequenced the complete mitochondrial genome—genetic information passed down from mothers—of a 38,000-year-old Neanderthal thighbone found in a cave in Croatia. (Get the basics on genetics.)

The new sequence contains 16,565 DNA bases, or "letters," representing 13 genes, making it the longest stretch of Neanderthal DNA ever examined.

Mitochondrial DNA (mtDNA) is easier to isolate from ancient bones than conventional or "nuclear" DNA—which is contained in cell nuclei—because there are many mitochondria per cell.

"Also, the mtDNA genome is much smaller than the nuclear genome," said study author Richard Green of the Max Planck Institute for Evolutionary Biology in Germany.

"That's what let us finish this genome well before we finish the nuclear genome," he said.

The new findings are detailed in the August 8 issue of the journal Cell.

Contrasted with modern humans, Neanderthals exhibited a greater number of letter substitutions due to mutations in their mitochondrial DNA, although they seem to have undergone fewer evolutionary changes overall.

The fact that so many mutations—some of which may have been harmful—persisted in the Neanderthal genome could indicate the species suffered from a limited gene pool. This might be because the Neanderthal population was smaller than that of Homo sapiens living in Europe at the time.